Estimation of near-surface attenuation in the tectonically complex contact area of the Northwestern External Dinarides and the Adriatic foreland

Abstract

Abstract. Seismic-induced ground motion at a site is generally influenced by seismic source, propagation path and local site conditions. Over the last several decades, researchers have consistently asserted that for near site attenuation, the spectral parameter kappa is subject primarily to site conditions. In this research we estimated parameter kappa based on the acceleration amplitude spectrum of shear waves, from the selected recordings of local earthquakes from seismological stations situated in the western part of Croatia from the slope of the high-frequency part. The spatial distribution of individual kappa values is compared with the azimuthal distribution of earthquake epicentres, with Vs30 values and the published coda-Q values for each station, as well as with isoseismal maps for several stronger events in the investigated area, along with the geological features. The dextral shift of crustal segments and frontal thrust of the External Dinarides along the Kvarner fault zone has probably had an impact on the geometry of the kappa parameter contour lines. These results are important for gaining further insight into the attenuation of near-surface crust layers in the Northwestern External Dinarides and the associated Adriatic foreland, as well as in similar geotectonic settings.

Active Tectonics in the Kvarner Region (External Dinarides, Croatia)—An Alternative Approach Based on Focused Geological Mapping, 3D Seismological, and Shallow Seismic Imaging Data

“…Since Adria is obviously generally moving to the north (Weber et al, 2010), a strain accumulated deeper in the crust is probably irregularly redistributed near the surface along the preexisting fault network formed during the earlier phases of the orogenesis. The discrepancy between seismologically constrained active faults and surface geology and geomorphology could be related to a complex shallow crustal anisotropy recognized generally also from the regionally differential attenuation of the seismic waves (Markušić et al, 2019). The sigmoidal strike of the generally orogen-parallel neotectonic near-surface possibly active faults ( Figure 12) could be related to the interaction of the deeper crustal longitudinal and transversal faults recognized in the 3D model according to the seismological data (Figure 8, Supplement 1 and 2).…”

“…FPS from previous studies in the area (Markušić et al, 2019) and from earthquakes in Croatian source mechanism database (Archive of the Department of Geophysics) were updated with FPS calculated within this research for the study area and magnitudes M ≥ 2.7 ( Figure 7). It should be noted that all events are weak-to-moderate earthquakes (M 2.7-4.5).…”

“…A system of small NE-SW and NNW-SSE striking transversal faults insignificantly dissected the fold limbs and were probably formed as conjugated faults during the folding. The predominantly steep faults strike along the fold axes and in FIGURE 7 | Focal mechanisms for the selected earthquakes (M ≥ 2.7) in the wider Bakar-Krk area and detailed information about the nine fault plane solutions (FPS) for the earthquakes (from the Croatian source mechanism database, an updated version of Markušić et al, 2019). Black areas on the lower focal hemisphere mark compression.…”